The curing and drying of cementitious products, such as cementitious concrete materials in the form of blocks, bricks and the like, is an old art dating back many centuries. The development of acceptable strength for such cementitious products is tied to chemical reactions which take place in the cementitious concrete. The primary chemical reactions that take place in such cementitious concrete products are the hydration reactions. The specific hydration reactions depend upon the source of the cement employed and, for the most part, involve four basic hydration reactions. These four basic reactions involve the hydration of (1) tricalcium aluminate, (2) tetracalcium aluminoferrite, (3) tricalcium silicate and (4) dicalcium silicate. Since the four reactions are hydration reactions, water must be mixed with or supplied to the cementitious concrete.
As used in the art, the term "curing" as applied to concrete products generally refers to the period between the molding operation and the time when the hydration reactions have been substantially completed during which hardening of the cement takes place through the chemical reactions between the cement and water. Regardless of the curing technique employed, the curing parameters of time, temperature and moisture are the most important and must be suitably balanced so as to maintain the concrete in the moist condition until the cement has properly hydrated or hardened. These parameters depend to some extent upon the selection of the concrete aggregates, the amount of cement used and the desired resulting properties of the concrete products, as is well known to those skilled in the art.
In addition to curing, drying of the cementitious material can also be important, especially in the production of cementitious block and brick. Generally, prior to the time the block or brick is able to be strapped together for shipping, it is dried and drying can be considered part of the overall "curing" process.
The oldest curing technique is natural curing wherein the concrete products are subjected to atmospheric conditions existing at the time the products are formed without special heating or wetting. The major drawbacks of natural curing are that the moisture needed for hydration of the cement is left to chance depending on weather conditions and that the curing process requires several weeks to 28 days. As a consequence and in view of the ever-increasing demand for concrete products, natural curing has for the most part been replaced by accelerated curing techniques.
There are several types of accelerated moist curing techniques currently in use, including vapor curing, low pressure steam curing all of which were free from application of liquid water and autoclave curing. In low pressure steam curing the concrete products to be cured are loaded into a kiln and a sufficient quantity of steam is injected directly into the kiln to heat the internal atmosphere thereof to the desired temperature while maintaining a high degree of saturation thereby providing the combination of heat and moisture which accelerates the hydration and hardening of the cement. Normally, saturated steam at atmospheric pressure is used to attain temperatures on the order of 100.degree. F. to 180.degree. F. or higher. In some instances, additional heat and moisture are directed to the kiln atmosphere by burning a gas and recirculating the products of combustion, with or without the addition of moisture. Vapor and autoclave curing are basically variations of low pressure steam curing.
After the moist curing operation such as with water vapor, low pressure steam, and autoclaving, the kiln, if drying is required and/or desired, is normally exhausted to expel the moisture laden atmosphere in preparation for the drying operation.
Typical drying methods include the use of butane or natural gas fuels for providing elevated temperatures in closed environments containing the cementitious material. The use of oil fired furnaces can result in staining of light-color concrete products. In any event, the forced warm air heating of the products consumes a considerable amount of butane, natural gas or other fuels occupying a considerable amount of time. Therefore, a need exists for a method of accelerated drying of cured cementitious material without utilizing butane, natural gas, oil or similar fuels.
The curing and drying of the block to a point where the block has sufficient strength for subsequent handling such as palletizing and/or strapping and shipping normally consumes a substantial amount of time even when accelerated with curing processes, such as with warm water, low pressure steam or autoclave curing methods. Therefore, a need exists for an improved method for curing and drying cementitious blocks and brick which reduces the time required for curing thereby allowing the material to be palletized and/or strapped, shipped and otherwise utilized within a shorter period of time.